Several different types of vehicle body components—such as doors, hoods, decklids, and liftgates, to name but a few—include two or more metal workpieces that are joined together. The metal workpieces may be joined at least in part by one or more resistance spot welds. These welds are usually formed around a periphery of the body component or at some other bonding region. In the past, the metal workpieces have typically been composed of steel, and for that reason spot welding practices have been specifically developed over many years with the particular aspects of spot welding steel to steel in mind. More recently, however, there has been a push to substitute aluminum alloy workpieces for steel workpieces wherever possible to try and reduce vehicle weight.
A resistance spot weld is generally formed by a stationary or robotically-moveable welding gun that includes two gun arms. Each of these gun arms holds a welding electrode typically comprised of a suitable copper alloy. The gun arms can be positioned on opposite sides of a workpiece stack-up and clamped to press the two electrodes against their respective metal workpieces at diametrically common locations. A momentary electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to the flow of electrical current through the metal workpieces and across their faying interface (i.e., the contacting interface of the metal workpieces) generates heat at the faying interface. This heat forms a molten weld pool which, upon stoppage of the current flow, solidifies into a weld nugget. After the spot weld is formed, the gun arms release their clamping force, and the spot welding process is repeated at another weld site.
The spot welding of a workpiece stack-up that includes an aluminum alloy workpiece can present peculiar challenges. For one, the aluminum alloy workpiece is usually covered by a variety of oxide layers (hereafter collectively referred to in the singular form as “oxide layer” for brevity) on its outer surface created both by processes experienced in mill operations (e.g., annealing, solution treatment, casting, etc.) as well as environmental exposure. This oxide layer increases electrical resistance at the contact patch. Because of the high electrical resistance of the oxide layer and the relatively low thermal and electrical resistance of the underlying bulk aluminum alloy, a high current density is typically required to form a weld pool at the surface of the aluminum alloy workpiece that forms a faying interface with the other metal workpiece in the stack-up.
While helpful in forming a weld pool at the desired location, a high current density can create excessive heat at the contact patch which, in turn, may accelerate a metallurgical reaction between the aluminum alloy that comprises the workpiece and the copper alloy that comprises the associated welding electrode. This reaction causes a contamination layer of copper-aluminum alloy to build-up or accumulate on the welding electrode. If left undisturbed, the contamination build-up can spall and form pits in the welding electrode, which ultimately harms welding performance and complicates electrode dressing. These complications, as well as others, present a variety of challenges regarding the design of a welding electrode that is intended to engage an aluminum alloy workpiece during spot welding, as opposed to some other type of metal workpiece, such as a steel workpiece.